1. Field of the Invention
The present invention relates to a method and an apparatus for controlling exhaust gas recirculation (EGR) in a diesel engine.
2. Description of the Prior Art
Usually, in EGR in a diesel engine, the more the amount of EGR increases the more the amount of hydrocarbon (HC) in the exhaust gas increases. On the other hand, if the amount of EGR is very small, a decrease in nitrogen oxide (NOx) in the exhaust gas cannot be expected. Accordingly, it is reasonable to determine the optimum amount of EGR so as to minimize the total weight of NOx and HC. On the basis of this, after an experiment was carried out by the inventors of the present invention, it became apparent that the rate of EGR in the engine must be decreased in accordance with an increase in the rotation rate of the engine. Consequently, it was found that the amount of EGR should be controlled so as to multiply the value of the rate of EGR illustrated in FIG. 1 by the correction coefficient C.sub.R, which represents the rate of reduction of the rate of EGR in accordance with the rotation rate, as defined in equation (1) below. EQU C.sub.R =-(1/4000)N+(5/4) (1)
where N is the rotation rate of the engine expressed by revolutions per minute (rpm).
The above-mentioned relationship is applied in an operating condition higher than approximately 1,000 rpm, and in an operating condition lower than approximately 1,000 rpm, it is desirable to reduce the rate of EGR in accordance with the reduction of the rotation rate because during idling or the like EGR causes delay of the ignition timing to increase. As a result, an increase in variation in the engine and an increase in noise is unavoidable.
In a conventional EGR apparatus, as mentioned above, the apparatus is not operated according to the corrected rate of EGR, i.e., the multiplied value of the rate of EGR by the correction coefficient C.sub.R, and, consequently, a satisfactory decrease in HC can not always be achieved.
The present invention is proposed in order to minimize the above-mentioned disadvantages.